Industrial wastewater and domestic wastewater are reused as industrial water after these wastewater are subjected to a treatment for removing organic matter and the like contained therein or are discharged to rivers and the like. As a treatment method of industrial wastewater or the like, for example, an activated sludge method can be exemplified. The activated sludge method is a method of decomposing organic matter and the like into aerobic microorganisms by aeration.
In recent years, a treatment according to a membrane separation activated sludge (MBR) method in which a treatment according to an activated sludge method and a membrane filtration using a separation membrane module are combined has been carried out. In the treatment according to the MBR method, since organic matter or the like accumulates on the surface of the separation membrane as the membrane filtration is continued, a decrease in filtration flow rate and an increase in transmembrane pressure difference occur.
With regard to such a problem, in the treatment according to the MBR method, the accumulation of organic matter on the membrane surface is prevented by an air diffusion pipe disposed below the membrane module. Specifically, the accumulation of organic matter on the membrane surface is prevented by the impact generated when the bubbles generated from the air diffusion pipe contact the membrane surface or the vibration of the membrane itself due to a water flow in accordance with the generation of bubbles.
From the viewpoint of the washability for the membrane surface, it is preferable to increase the size of the bubble. Patent Literatures 1 and 2 disclose a siphon-type air diffusion pipe which efficiently discharges large bubbles. Further, Patent Literature 3 discloses a method of supplying a gas from a plurality of gas outlets of a distribution pipe disposed above or below a plurality of discharge guide pipes (corresponding to siphon-type air diffusion pipes) and intermittently discharging bubbles from a plurality of discharge guide pipes.
Further, Patent Literature 4 discloses a membrane separation unit capable of performing an intermittent aeration operation with a simple configuration without requiring a particular control system and increasing an electric cost as compared with a continuous aeration operation.
When the siphon-type air diffusion pipe is used to wash the membrane module, the siphon-type air diffusion pipes are arranged in use in response to the size and the number of the membrane modules. In order to supply air to each siphon-type air diffusion pipe, a method of directly connecting a plurality of introduction pipes branched from a distribution pipe to an air supply port of an upper portion of each siphon-type air diffusion pipe or disposing a single pipe with a hole directly below each siphon-type air diffusion pipe to send air into each siphon chamber is used.